A monochromic thermal printer is known which uses as the inking medium a length of relatively broad strip of a film coated with heat-sensitive ink. A standardized version of such an inking medium has a width of about 210 mm and a thickness within the range of from about 10 microns to 30 microns. Because of its width which is relatively large for the thickness, an inking medium of this nature tends to produce wrinkles and to locally deviate on a record sheet or printing paper and can not be wound on a take-up roll smoothly and uniformly.
In an attempt to provide a solution to this problem, it has been proposed to use a thermally activated inking medium of ribbon form having a reduced width of, typically, from about 10 mm to 20 mm as a substitute for the prior inking medium in the form of a broad strip. A thermal printer using such an advanced heat-sensitive inking medium is disclosed in, for example, Japanese Provisional Patent Publication No. 55-55883. The heat-sensitive ink ribbon is coated or impregnated with ink of, usually, black color throughout its length and, for this reason, the thermal printer using the ribbon is not operable for printing patterns of information in different colors.
The applicant has, accordingly, proposed a novel multicolor thermal printing apparatus which exploits all of the advantages attainable by the prior-art thermal printers using a single-colored ink ribbon of the reduced width. This printing apparatus is disclosed in co-pending U.S. patent application Ser. No. 675,393 filed 27th Nov., 1984. The multi-color thermal printing apparatus disclosed therein uses as a printing medium a continuous, variegated heat-sensitive ink ribbon which is coated or impregnated with inks of different colors. The ink ribbon has a series of recurrent, discrete colored sections which are contiguous to one another throughout the length of the ribbon and which consist of first colored sections inked in a first color such as yellow, second colored sections inked in a second color such as magenta and third colored sections inked in a third color such as cyanic blue. The first, second and third colored sections occur, lengthwise of the ribbon, successively and recurrently with a unit series consisting of one first colored section, one second colored section subsequent to the first colored section, and one third colored section subsequent to the second colored section. The first and third colored sections of each unit series are respectively subsequent and preceding to the third and first colored sections of the immediately preceding and subsequent unit series.
During printing operation using such a heat-sensitive parti-colored ink ribbon, the ribbon is driven to travel along an array of heater elements forming a printing head and is caused to frequently stop and restart travelling at predetermined time intervals. In this instance, difficulties are experienced in enabling the ink ribbon to stop in correct positions with respect to the array of the heater heater elements of the printing head. Furthermore, the ink ribbon, which is susceptible to changes in tension and ambient temperature, tends to be caused to shrink over some areas and elongate over other areas during operation of the printer. The local shrinkage and elongation of the ribbon results in fluctuations in the lengths of the individual colored sections of the ribbon and makes it difficult for the ribbon to have the individual colored sections located correctly in registry with those sets of heater elements of the printing head which should be associated with the respective colored sections during each dot printing step. It may thus happen that some or even all sets of heater elements of the printing head are brought into registry with longitudinal portions of the ink ribbon which contain the boundaries between the adjacent colored sections of the ribbon. When a boundary between any adjacent two colored sections of the ink ribbon happens to be located between those two sets of heater elements which should be respectively located in registry with these two colored sections, the dots which should have been printed in a certain color by one of these two colored sections will be printed some in one color and the others in another. This results in unintended distribution of colors in the printed information pattern and possibly further in indistinctness of the pattern from the environment of the record sheet.
The present invention contemplates resolution of these problems. It is, accordingly, an important object of the present invention to provide a novel multicolor thermal printing method and an improved multi-color thermal printing apparatus which are useful for avoiding unintended, objectionable distribution of colors in printed patters of information and for forming printed patterns of information with clear-cut contours even when the ink ribbon may have failed to have some of its colored sections located correctly with respect to the heater elements of the printing head.